Stiffness Tuning with Bias Magnetic Fields in Ferromagnetic Shape Memory Ni-Mn-Ga

This article presents the dynamic characterization of mechanical stiffness changes under varied bias magnetic fields in single-crystal ferromagnetic shape memory Ni-Mn-Ga. The material is first converted to a single variant through the application and subsequent removal of a bias magnetic field. Mechanical base excitation is then used to measure the acceleration transmissibility across the sample, from where the resonance frequency is directly identified. The tests are repeated for various longitudinal and transverse bias magnetic fields ranging from 0 to 575 kA/m. A SDOF model for the Ni-Mn-Ga sample is used to calculate the mechanical stiffness and damping from the transmissibility measurements. An abrupt resonance frequency increase of 21% and a stiffness increase of 51% are obtained with increasing longitudinal fields. A gradual resonance frequency change of -35% and a stiffness change of -61% are obtained with increasing transverse fields. A constitutive model is implemented which describes the dependence of material stiffness on transverse bias magnetic fields. The damping exhibited by the system is low in all cases (approximate to 0.03). The measured dynamic behaviors make Ni-Mn-Ga well suited for vibration absorbers with electrically tunable stiffness.